1. Field of the Invention
The present invention relates to a new process for the preparation of aqueous dispersions or solutions of cross-linkable polyurethane polyureas containing cross-linking agents which can be activated by heat, to the dispersions or solutions obtainable by this process and to their use for the production of coatings which have a high resistance to water and solvents, in particular on metal substrates.
2. Description of the Prior Art
Processes for the preparation of aqueous polyurethane polyurea dispersions are known. Those dispersions which are prepared on the principle of incorporating hydrophilic centers (so-called internal emulsifiers) in the macromolecular chain of the polymer (e.g. German Pat. Nos. 1,184,946 and 1,178,586, German Auslegeschrift No. 1,237,306 and German Offenlegungsschriften Nos. 1,495,745, 1,595,602, 1,770,068 and 2,019,324; see also D. Dieterich et al, Angew. Chem. 82, 53 (1970)) are particularly important on account of their exceptional stability and the comparatively high water-resistance of the coatings obtained from them.
Whereas some important manufacturing processes for aqueous polyurethane urea dispersions are dependent on the use of organic solvents, with the disadvantage that these solvents must be removed from the dispersions by expensive distillation processes to be subsequently rectified, other processes are known which enable polyurethane ureas to be prepared in the aqueous phase with little or no solvent.
Particularly elegant processes for the solvent-free preparation of aqueous dispersions or solutions of polyurethane polyureas have been described in German Offenlegungsschrift No. 2,725,589 (corresponding to U.S. application Ser. No. 908,885 of May 24, 1978), German Offenlegungsschrift No. 2,811,148 or U.S. Pat. No. 4,269,748 and German Offenlegungsschrift No. 2,732,131 or U.S. Pat. No. 4,192,937. In these processes, prepolymers which contain isocyanate end groups and which have been hydrophilically modified, i.e. which contain built-in ionic centers and/or external emulsifiers, are mixed with chain lengthening agents in a blocked form which give rise to polyamines or hydrazines under the hydrolytic influence of water. When the resulting mixtures are subsequently mixed with water, the formation of stable aqueous dispersions or solutions of polyurethane polyureas takes place, accompanied and/or followed by a chain lengthening reaction of the isocyanate groups with the hydrolytically formed polyamine or hydrazine.
These processes result in polyurethane ureas which are equal to the products produced by the conventional "solvent process" in their mechanical strength and resistance to hydrolysis and even surpass them in some characteristics which are important for their practical application. The dispersions obtained therefore satisfy the standards of quality required for numerous applications, e.g. for textile coating, where the high flexibility of the substantially linear polyurethane ureas is an advantage.
When non-flexible substrates such as metals are to be coated, it is necessary for certain purposes to use highly cross-linked polymers. This is frequently the case when the lacquers are required to have very high resistance to solvents and other chemical agents and high temperature resistance.
Polymers obtained from the aqueous phase are already in use to some extent even for these fields of application. The polymers used for this purpose are in most cases stoving lacquers based on combinations of polyester or polyacrylate dispersions and, for example, melamine/formaldehyde resins. It would be desirable, however, if systems which have the typical advantages of polyurethane lacquers obtained from organic solution, such as high elasticity and abrasion resistance, excellent resistance to weathering and very high water and solvent resistance were also available in the field of aqueous stoving lacquers.
It was therefore an object of the present invention to provide a solvent-free or low solvent process for the preparation of storage stable aqueous dispersions or solutions of cross-linkable polyurethane polyureas containing cross-linking agents capable of being heat activated, which would be suitable for the production of coatings with improved mechanical properties and increased water-resistance and resistance to hydrolysis.
This problem could be solved by providing the process according to the invention described below which is based on the surprising observation that if, in the processes known in the art for the preparation of aqueous dispersions or solutions of polyurethane polyureas, a mixture of hydrophilically modified isocyanate prepolymers and blocked polyamines or blocked hydrazines are mixed with water, it is possible to incorporate hydrophobic polyisocyanates containing blocked isocyanate groups in the mixture which is to be mixed with water without thereby precipitating the above-mentioned hydrophobic component. This observation was unexpected since it is known, for example, from German Auslegeschrift No. 2,624,442, that in order to incorporate hydrophobic cross-linking agents, in particular blocked polyisocyanates, in previously prepared aqueous polyurethane dispersions, it is necessary to add hydrophilic solvents as solubilizing agents or to use hydrophobic additives in an emulsified form. The possibility discovered according to the invention of incorporating hydrophobic cross-linking agents in the dispersions or solutions without at the same time using hydrophilic solvents or additional emulsifiers is of considerable technical interest since the omission of the above-mentioned hydrophilic auxiliary agents will, of course, increase the water-resistance and resistance to hydrolysis of the sheets or coatings obtained from the dispersions or solutions. For the same reason, the end products according to the invention which are described below are superior to the known aqueous polymer dispersions which contain water-soluble or dispersible blocked polyisocyanates as cross-linking agents (see e.g. German Offenlegungsschrift Nos. 2,456,469 and 2,708,611, European Offenlegungsschrift No. 12,348 or European Offenlegungsschrift No. 22,452).
The aqueous dispersions or solutions obtained by the process according to the invention which is described more fully below also have other unforeseeable technically advantageous properties. Thus they are particularly suitable for the production of stoving lacquers which combine excellent mechanical characteristics with a surprisingly powerful protection against corrosion on metal substrates, even on those which have not been pretreated (e.g. phosphatized). Corrosion protective coatings on metal substrates which have not been pretreated are of considerable technical interest. Another feature which was not foreseeable is that these stoving lacquers manifest their advantageous properties even when they have been hardened at comparatively low temperatures, and the dispersions or solutions obtained from the process according to the invention have an excellent stability against sedimentation even at the elevated temperatures which may occur in transport or storage. It should be particularly emphasized that for the purpose of ensuring that the films or coatings obtained from the dispersions or solutions will have the above-mentioned characteristics, it is not necessary to incorporate special isocyanate reactive groups such as hydroxyl or carboxyl groups in the basic polyurethane polyurea molecule although such groups may, of course, be present.